I. Field of the Invention
The present invention relates to data communications. More particularly, the present invention relates to method and apparatus for efficiently demodulating signals that have been processed and transmitted in a diversity mode.
II. Description of the Related Art
In a typical digital communications system, data is processed, modulated, and conditioned at a transmitter unit to generate a modulated signal that is then transmitted to one or more receiver units. The data processing may include, for example, formatting the data into a particular frame format, encoding the formatted data to provide error detection and/or correction at the receiver unit, channelizing (i.e., covering) the coded data, and spreading the channelized data over the system bandwidth. The data processing is typically defined by the system or standard being implemented.
At the receiver unit, the transmitted signal is received, conditioned, demodulated, and digitally processed to recover the transmitted data. The processing at the receiver unit is complementary to that performed at the transmitter unit and may include, for example, despreading the received samples, decovering the despread samples to generate decovered symbols, and decoding the decovered symbols.
In some communications systems, data is processed and redundantly transmitted over two (or possibly more) antennas to provide transmit diversity. The processing may include, for example, covering the data for each antenna with a particular channelization code (e.g., a particular Walsh symbol). In some systems, the data for one or more antennas may also be reordered prior to the channelization. Due to multipath and other phenomena, the transmitted signals may experience different path conditions and may arrive at the receiver unit at different times. If the transmit antennas are spaced sufficiently far apart, then the received signals from the antennas tend to fade independently. Each transmitted signal may also reach the receiver unit via multiple signal paths. The receiver unit is then required to receive, track, and process one or more instances of each transmitted signal, and to combine the results from the processed signal instances to recover the transmitted data. On the downlink, the processing typically includes tracking a pilot that has been transmitted along with the data, and using the recovered pilot to demodulate data samples.
The signal processing (e.g., demodulation) to process multiple transmitted signals, and multiple instances of such signals, can be complicated. Moreover, transmit diversity is typically provided on the downlink, and user terminals are required to support such a mode. The user terminals are typically more impacted by complexity and costs considerations. Therefore, techniques that can be used to efficiently demodulate signals that have been processed and transmitted in a diversity mode are highly desirable.